Electrical machines, such as motors and generators having a rotor with permanent magnets are known. They are generally deemed to be reliable and require less maintenance than other generator topologies.
Modern wind turbines are commonly used to supply electricity into the electrical grid. Wind turbines of this kind generally comprise a rotor with a rotor hub and a plurality of blades. The rotor is set into rotation under the influence of the wind on the blades. The rotation of the rotor shaft either directly drives the generator rotor (“directly driven”) or through the use of a gearbox. Particularly for offshore wind turbines, direct drive systems employing permanent magnets are usually chosen.
Such a direct drive wind turbine generator may have e.g. a diameter of 6-8 meters and a length of e.g. 2-3 meters. Hundreds of magnets may need to be attached, e.g. by screwing or gluing to the circumference of the rotor. It will be clear that this manufacturing process can be cumbersome.
Furthermore, if one or more magnets is damaged and needs to be replaced, the access to these magnets can be complicated (especially with large size generators), such that maintenance can become very expensive.
The permanent magnets may be made e.g. from AlNiCo steel (Aluminium-Nickel-Cobalt) or rare earth magnetic materials such as neodymium (NdFeB), or samarium-cobalt. These materials may be relatively expensive. For this reason, it is important that the permanent magnets have the minimum size required and that loss of magnetic flux of the permanent magnets of the rotor is reduced to a minimum.
Cooling of the components is generally an important requirement in electrical machines. A cooling air flow may be established by the rotation of the rotor. In some implementations, an additional cooling air flow may be provided by a dedicated fan. Furthermore, in some implementations a liquid cooling circuit may be provided. Incorporation of a cooling circuit may further complicate the assembly and manufacture of large permanent magnet generators. A plurality of radial cooling holes may need to be drilled. These radial holes may complicate manufacture and assembly and will generally not lead to homogeneous cooling.
This size and type of generator however is not limited to offshore applications, and not even to the field of wind turbines only. Generators of considerable dimensions that may suffer from the same problems and/or have the same complications may also be found e.g. in steam turbines and water turbines. Also relatively large permanent magnet motors may have the same or similar problems.